Electron lens system



llg 31, 1954 BUNYA TADANo E-rAL ELECTRON LENS SYSTEM 2 Sheets-Sheet l Filed May 17, 1952 Fi9.l

l A mm nas liv/fs Aug. 3l, 1954 BUNYA TADANo ETAL 2,688,092

ELEcTRoN LENS SYSTEM Fileduay 17, 1952 2 sheets-sheet 2 H ww wm Patented Aug. 31. 1954 ELECTRON LENS SYSTEM Bunya radano, Nozema Morito, and Shnjiro Katagiri, Kita-Tama-Gun, Iokyo, Japan, assignors to Hitachi Limited, Chiyoda-Ku, Tokyo,

Japan Application May 17, 1952, Serial No. 288,382

Claims priority, application Japan May 21, 1951 l (liaim.

The present invention relates to improvements in electron lens systems, and more particularly to a special selection of the characteristics of cooperating electron lenses in an electron miercscope.

The object of this invention is to provide an electron lens system for electron microscopes and electron diffraction apparatus etc., which has the least chromatic eld aberration, compounded of chromatic difference in. magnification and chromatic difference in rotation irrespective of the variation or irregularity of electron speed or exciting magnetic eld.

Another object oi this invention is to provide an electron lens system for electron microscope, which can satisfy the condition for eliminating chromatic eld aberration at lower exciting ampere turns, by which the chromatic eld aberration, compounded of chromatic difference in magnification and chromatic difference in rotation can be completely or substantially eliminated over a wide range of magnification.

This invention will be better understood from the following description taken in connection with the accompanying drawing, in which Fig. 1 shows the characteristic curves of the coeicient of chromatic diierence in magnification of an objective;

Fig. 2 shows a similar characteristic curve to Fig. l of a projector;

Fig. 3 shows a characteristic curve of the coelicient of chromatic difference in rotation of an objective;

Fig, 4 shows a similar curve to Fig. 3 of a projector;

Fig. 5 is a diagrammatic sectional elevation of an electron microscope taken for the explanation of this invention;

Fig. 6 shows characteristic curves of coeicients of chromatic aberration of magnication and rotation of magnetic lenses;

Fig. 7 is a diagrammatic sectional elevation of an electron microscope showing an embodiment of this invention;

Fig. 8 is an explanatory diagram for illustrating the manner of image formation in the lens system as shown in Fig. '7, and

Fig. 9 shows curves of coeicients of chromatic aberration of magnication and rotation of an intermediate lens when used under a specific condition embodying this invention.

In general, the chromatic dilTerence in magnication of an objective and that of a projector in an electron microscope are expressed by the following equations:

(Cl. 25d-495) 2 wherein E represents the voltage speed of electron in volts; AE the variation of the voltage speed; 'J the distance from the optical axis to the object point; Coy and Cpy represent the coefcient of chromatic difference in magnication of an objective and a projector respectively. The inventors have ascertained after careful investigations and numerous experiments that these co'. efficients have such characteristics as shown by curves A and B in Figs. 1 and 2 respectively in case of magnetic lenses. In these gures, and in the following description and claims, IN represents ampere turns of the coil of lens or equivalent ampere turns of the permanent magnet alone or the resultant eld of it and the exciting coil.

The coeicients Coy and Cpy have heretofore been theoretically considered to followsuch a curve .C as shown in Fig. 1, which can never take zero or positive value.

The present invention is based on the discovery of the characteristics that these coefficients vary according to the curves A and B in Figs. l and 2 as the ratio INA/ is changed, that is, the coefficients increase as IN/\/E is increased, pass through zero, and become positive. Such characteristics are used to eliminate the chromatic aberration in electron lens systems consisting of a plurality of lenses.

Fig. 5 shows an example of electron microscope having the lens system composed of a magnetic objective i and a projector 2. In the iigure, 3 designates an electron source; Il a condenser; 5 a specimen; 6 a viewing window; l a iiuorescent screen; and 8 a photographic plate.

In this case, the voltage speed E of the electron beam b projected into each lens I and 2, and the ampere turns 1N of their exciting coils l and 2 are selected to appropriate values, such cordance with the variationv of INA/ `asshownv by the curves D and E in Figs. 3 and i as for in-` stance, ii it is desired to eliminate the chromatic aberration of magnication and rotation, the values of IN/x/E for each lens should beselected so as to eliminate also the chromatic aberration of rotation.

As for example, when the coeicientCoy for a certain value of IN/V of the objective I is neg 3 ative on the curve A in Fig. 1 and the coeiflcient Cox is positive on the curve D as shown in Fig. 3, if the image rotation of the projector 2 is made reverse to that of the objective I to cause the chromatic aberration of rotation to vanish, Cpy and Coy would not inevitably have equal values and opposite signs for the same value of INA/E, when Cpx on the curve E in Fig. 4 is so selected that CpI=C"01. When both of these conditions, that is, the condition of that C'0e=C'px, and Coy is equa1 and opposite to Cpy can not be fulnlled even though the values of IN/\/E of these two lenses I and 2 are regulated properly, then the ratio h/d of the pole piece spacing to the pole piece diameter of the projector 2 may be regulated for each value of IN /V to make C'0z=C'p, thereby changing Cpy to become equal and opposite to C'ey, owing to the fact that the zero position of Cpy on the curve B in Fig. 2 will be shifted by the change of the ratio h/d, while Coy, Cox and Cpx are less aiected by the change of h/d. This phenomenon is used to make Cpy equal and opposite to Coy by regulating the value of h/d of the projector 2 at each value of IN /\/E when C '01:0 'p r.

Very high magnification can be obtained by an electron microscope, but in practice there occur pretty many cases which need to compare the magnified image of the electron microscope with that obtained by an optical microscope. In such cases, it is necessary that the magnification of the electron microscope be variable over a very wide range for instance from 1000 to 10,000 times. However if, in order to vary the magnification,

the value of the IN/\/E of magnetic lens is changed, the values of the coefiicient of chromatic difference in magnification of an objective, Coy, and that or" a projector and other magnetic lens, Cpy, and also the coefficients of chromatic difference in rotation Cox and Cpx respectively would vary according to the curves shown in Fig. 6. Accordingly even though the chromatic aberration is caused to vanish at a certain magnification, that is, at a certain value of INA/E, the condition of mutual cancellation of chromatic aberrations will deviate when the magnification is changed from a predetermined value so that it is impossible to always keep the chromatic aberration at a small value over a wide range of magnication.

For this purpose, according to this invention, the chromatic aberrations of electron lenses can be maintained always at a very small value without interchanging the lens even when it is required to vary the magniication over a wide range. Fig. 7 shows an electron lens system for attaining the above object embodying this invention, wherein I designates an objective; 9 an intermediate lens, and 2 a projector. The intermediate lens 9 is used at a magnification less than unity as understood from Fig. 8 which illustrates diagrammatically the formation of the image of a specimen 5 by the magnetic lens system. In Fig. 8, the paths of electron beam are shown by the full line Ia., and i and i' designate intermediate and final images respectively. It will be understood from Fig. 8 that the magnication for the nal image i' of the specimen may be changed by varying the strength of the intermediate lens 9.

The coeicient of chromatic difference in magnification Cmy of the intermediate lens 9 varies according to its distance to the objective I and to the projector 2, and also varies according to the value of IN/\/E on the curve Cmy in Fig. 9. On the other hand, the coeicient of chromatic difference in rotation Cmx varies relative to IN /\/E similarly to that of the projector. It will be understood from Fig. 9 that when the intermediate lens 9 is used at the magnification less than unity, the value of Cmy is always positive and has the tendency of rapidly increasing as INA/ is increased. The coefficients of chromatic difference in magnification, Coy and Cpy, and the coecients of chromatic dinerence in rotation, Cox and Cpx of objective I and projector 2 respectively have characteristics as shown by the curves in Fig. 6 for example, and Coy and Cpy take negative values for the lower range of INA/', while their absolute values decrease as INA/' is increased and pass through the zero line and rapidly ascend to positive.

Now assuming the intermediate lens 9 is not operated and if the IN/\/E of objective I and projector 2 be taken at 19 and 12.5 respectively, then the following relations will establish from Fig. 6:

CpyO CayO If these lenses are excited in opposite directions, Cpe-Correo and also Cpy-i-C'oy, thus the object of eliminating the chromatic aberration can be attained.

Nert if the intermediate lens 9 is operated and IN /\/E is taken at 1.5, then the following relations will result from Fig. 9:

li INA/ of the projector be 10, then Gau- 0.45 Cpro'l It is a well known fact to those skilled in the art that if the value of IN/\/E of an objective and the position of the microscope screen are given, the specimen should be located at a definite position along the optical axis with respect to the given objective. If in this case the geometrical position of the specimen relative to the objective is not changed, the value of INA/E, at which the focus coincides, is unchanged from the condition as above mentioned and is maintained at 19 so that Accordingly the total sum of coefcients of chromatic diierences in magnication and rotation will be Thus the algebraic sum of the coeicients of chromatic dierences of the lens system will become substantially small. In this case, the directions of excitation of the intermediate lens and projector are the same, and opposite to that of the objective. Thus the magnification obtained for the rinal image is 7,000 times, which was 10,000 times before the reduction of magnification.

u If we select the value of INA/'' of the intermediate lens at 3, that of the projector at 8 and that of the objective at 19, then from Figs. 6 and 9 we can obtain The results show that the chromatic difference in rotation can be disregarded if compared with the values when no precautions were taken to eliminate the chromatic aberrations. The iinal magnication in this case is about 3,000 times. The final magnification range may be changed for instance from 3,000 to 1,000 by proper selection of the projector 4.

Though we have described in the foregoing an example of lens system consisting of both objective and projector, yet the use of an intermediate lens for reducing the magnification according to this invention can equally be applied to other compound lens systems consisting of more than three lenses either objective or projector only, or involving an electrostatic lens.

It will be clear from the foregoing description that the present invention has the excellent eiect of eliminating or reducing the total chromatic aberration in magnification and chromatic difference in rotation for electron lens systems irrespective of the variation of electron speed and/or exciting current over a wide range of magnication.

In our co-pending application Serial No. 288,381 Iiled of even date herewith, on the contrary, we have disclosed an invention which eliminates the chromatic aberration in an electron lens system having a narrow or xed range of magnification.

What we claim is:

In an electron microscope, an electron lens system comprising in combination an electron source, a condenser, a magnetic objective having a value of INA/ selected from 16 to 22, a specimen holder located at the focal plane of said objective, a magnetic intermediate lens having a magnification less than unity and having a value of IN/\/E selected from 0 to 5, and a magnetic projector including pole pieces having a ratio of mutual spacing to diameter ranging from 0.5 to 5, and IN /\/E selected from 8 to 19, wherein a value of IN represents the equivalent ampere turns of the exciting eld of the lens system and E the voltage speed of electrons in volts, all the above elements being arranged successively along the path of electron beam, means for exciting said obj ective oppositely to the excitation of said intermediate lens and said projector, and means for adjusting the excitation of said intermediate lens relatively to said projector so as to eliminate all chromatic field aberration compounded of chromatic diierences in magnification and in rotation over a wide range of magnification of the electron microscope.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,233,264 Marton Feb. 25, 1941 2,270,112 Borries et al J an. 13, 1942 2,354,287 Zworykn et a1. July 25, 1944 2,485,754 Le Poole Oct. 25, 1949 2,547,994 Bertein Apr. l0, 1951 OTHER REFERENCES A Study of Distortion in Electron Microscope Projection Lens, Hillier Journal of Applied Physics, vol. 17, June 1946, pages 411-419. 

